Digital printing machines can take on a variety of configurations. One common process is that of electrostatographic printing, which is carried out by exposing a light image of an original document to a uniformly charged photoreceptive member to discharge selected areas. A charged developing material is deposited to develop a visible image. The developing material is transferred to a medium sheet (paper) and heat fixed.
Another common process is that of direct to paper ink jet printing systems. In ink jet printing, tiny droplets of ink are sprayed onto the paper in a controlled manner to form the image. Other processes are well known to those skilled in the art. The primary output product for a typical digital printing system is a printed copy substrate such as a sheet of paper bearing printed information in a specified format.
The output sheet can be printed on one side only, known as simplex, or on both sides of the sheet, known as duplex printing. In order to duplex print, the sheet is fed through a marking engine to print on the first side, then the sheet is inverted and fed through the marking engine a second time to print on the reverse side. The apparatus that turns the sheet over is called an inverter.
FIG. 1 shows a state-of-the-art inkjet digital printing machine 40. Printer 40 includes a marking module or engine 42 having an ink jet print head or multiple print heads 43, disposed centrally on the marking engine 42, and facing downward. Printer 40 has a media path or process path 44 along which the media sheet 54 moves. Printer 40 has a media path entrance 46 where sheets are fed into the printer by a media sheet feeder (not shown). Printer 40 also has a media path exit 48 where sheets leave the printer and are fed into a finisher (not shown). Printer 40 has an inverter 50 to turn the sheet over for duplex printing. A media sheet 54 leaving the inverter 50 follows arrow 52 back to the marking engine 42 for printing on the reverse side. Arrow 44 indicates the process path direction, which is downstream from entrance 46 toward exit 48. A vacuum transport conveyor 60 moves the media sheet 54 under the print head 43.
FIGS. 2-5 show that in cut sheet printing devices, under certain conditions, the lead-edge of the paper can curl up and have potential for separating from the marking transport and contact the print head. A sheet 54 with out-of-spec flatness can occur when a duplexed sheet has a heavy ink image on the trail edge 58 of side 1, which then becomes the lead edge 56 when inverted and curls towards Side 2. This is most severe when the paper is thin, the aqueous ink coverage is heavy, there is a border bar image near the lead edge, and the cross-process direction image is parallel to the grain direction of the paper (Example: letter size paper, grain-long, long-edge-feed).
In direct-to-paper ink jet marking engines, an ink jet print head 43, or typically multiple ink jet print heads 43, are mounted such that the face (where the ink nozzles are located) of each print head is mounted a fixed distance from the surface of the media 54. The gap is typically 1.5 mm or less. Because the paper curl height can be several millimeters, it poses a risk to the print head because the paper can hit the print head face plate when it passes through the nominally thin gap that the print heads are spaced from the media.
Media sheets 54, typically paper, can curl or distort in several ways. LE curl is a concave upward bending along the process direction, such that the lead edge 56 (LE) and the trail edge 58 (TE) rise up off the transport 60, as shown in FIG. 2. The raised LE can impact multiple print heads across the paper width. Cross curl is a concave upward bending across the process direction, such that the left side and right side edges rise up off the transport, as shown in FIG. 3. The raised sides can impact multiple print heads. Both LE curl and cross curl are caused by ink on the first side of a duplex print that is inverted.
Dog ear is a crease with upward bending across the process direction at an angle across a corner, as shown in FIG. 4. The crease can impact multiple print heads downstream. This is caused by sheet damage in the paper path. Print head damage is severe due to greater pressure.
Cockle is multiple bumps or peaks distributed throughout the sheet, as shown in FIG. 5. The bumps can impact multiple print heads downstream. Cockle is caused by the drying rate of ink, especially aqueous based inks.
For ideal image quality, the print head gap or distance of the print head 43 to the sheet 54 should be maintained at less than 1.2 mm, preferably within 1 mm. The media sheet traveling at one meter per second must pass freely under the print heads. The sheet must not contact the face of the print head, or serious damage will result. This requirement poses a challenge for cut sheet media since the corners, edges and body of the sheet may not be completely flat. The use of a hold down transport such as a vacuum conveyor helps to maintain the sheet flat and within the gap for the most part. Purposely delivering sheets with downward curl from the sheet supply tray also helps to hold the sheet flat. Nevertheless it is not guaranteed that a sheet is flat over the entire surface.
Ink jet print heads are very delicate and can easily be damaged if the face of the print head is contacted by the media which is passing nearby. The print heads are also very expensive. Thus, it is very important to minimize any risk of damaging these print heads.
Accordingly, there is a need to provide a print head protection device for inkjet printers that will prevent print head damage, and also prevent jamming of sheets on the vacuum transport conveyor.
There is a further need to provide a print head protection device for inkjet printers of the type described and that will match the high production rate of a digital printing machine.
There is a yet further need to provide a print head protection device for inkjet printers of the type described and that is mechanically simple and robust, thereby minimizing cost.